A centerboard, a plate sliding through a slot in the hull, performs best but requires cutting the hull, while a leeboard, one hung over each side, bolts on and lowers on the leeward side when sailing upwind; either must produce a sideways force equal to the sail’s. For 382 N at a boat speed of 2 m/s a 0.6 square meter board needs a lift coefficient of about 0.32, which implies at least about 2.9 degrees of leeway even in the ideal case, and a leeboard hung 0.6 m below its pivot loads a chock 0.25 m below the pivot with about 917 N, 2.4 times the water force.
Part of Small Sailboat Construction
A centreboard or leeboard is the underwater fin that lets a sailboat make headway across and against the wind by resisting the sideways push of the sail.
The Board Works By Sailing Slightly Sideways Through The Water
A board produces sideways force only if the water meets it at an angle, the angle of attack. That angle is the boat’s leeway. A larger board needs less angle for the same force, and a sharper-edged, smoother board produces force more efficiently. The balance fixes how much of the boat’s progress is lost to slipping sideways, and it can be computed to a first approximation.
The carrier’s description is: centreboard (slides through a slot, best performance but modifies the hull); leeboard (hung over the side, bolted or clamped on, lowered on the leeward side when sailing upwind); board length about a third to a half of the boat, extending 40-60 cm below the hull.
What You Need
- A board of dense, straight-grained wood, or a metal plate (the latter doubles as low ballast)
- For a centreboard: a trunk (a boxed slot), a pivot pin, and a lifting line
- For leeboards: a pivot bolt through a stout cleat at each gunwale, a chock below it against the hull, and a lifting line
- Sealant for the trunk or bolt holes (see Boat Sealing)
- A way to measure speed over a known distance
Method: Size The Board And Check The Attachment
Step 1: Fix the sideways load. Take the sail force at the top of force 5, 382 N (the 5.6 m² sail in Spritsail Rig). Treat the whole force as sideways: this overstates it, which is the safe direction.
Step 2: Compute the lift coefficient for a candidate board. With water density 1,000 kg/m³ and boat speed 2 m/s, dynamic pressure is ½ × 1,000 × 2² = 2,000 N/m². For a board of 0.6 m² (1.5 m long, 0.4 m deep, from the carrier’s size), C_L = 382 ÷ (2,000 × 0.6) = 0.32. For 1.35 m² (2.25 m by 0.6 m), C_L = 0.14.
Step 3: Convert to a minimum angle. The lift slope of an ideal thin foil of infinite span is 2π per radian, about 0.11 per degree (a textbook figure, recalled, not measured here). Leeway is at least C_L ÷ 0.11 = 2.9° for the small board and 1.3° for the large one. A real board of short span has a lower slope, so its leeway is larger. The calculation sets a floor, not a prediction.
Step 4: Convert the leeway into lost progress. At 3 m/s, heading 45° off the wind gives 3 cos 45° = 2.12 m/s upwind. With a leeway of 3° the track is at 48°, giving 3 cos 48° = 2.01 m/s, a loss of 5%.
Step 5: Design the leeboard attachment (below) and test it with the boat afloat and a load on the board.
Worked Example: Loads On A Leeboard Mounting
Water pushes the lowered lee board toward the hull with F = 382 N, acting about 0.6 m below the pivot. A chock on the hull side 0.25 m below the pivot reacts. Taking moments about the pivot, the chock force R satisfies R × 0.25 = 382 × 0.6, so R = 917 N, which is 917 ÷ 382 = 2.4 times the water force. The pivot bolt takes the rest of the balance: 917 − 382 = 535 N. These two must be strong: the chock presses on the planking and the bolt pulls on the cleat. Spread the chock over a wide pad or over a rib (see Ribs & Frames) instead of a single plank.
Common Mistakes
- A leeboard on a thin plank — the 917 N chock load crushes or punches through light planking.
- A centreboard trunk open at the top — water floods in when the boat heels; the trunk top must be above the waterline.
- A floating board — a light wooden board rises on its own; weight it or fit a downhaul.
- Forgetting to swap boards on a tack — only the leeward board is lowered when sailing upwind (per the carrier), so each tack means raising one and lowering the other.
Safety
Boards can trap and flood
A trunk under the floor is a hole in the boat; a board that jams in the slot after grounding is a leak you cannot fix afloat. Use a pivot and a stop so the board kicks up when it strikes bottom, and keep a wooden plug that fits the slot in case the board must be removed.
What’s Next
- Learn next: Rudder & Tiller — the other underwater part
- Combine with: Points of Sail — how the board and the wind angle trade against each other